In the past, chromium-containing passivating layers were used predominantly to prevent the corrosion of metallic materials. In this connection, the particularly poisonous and carcinogenic yellow chromating was displaced increasingly by the somewhat less hazardous green chromating. The very good corrosion-inhibiting properties of such coatings based on chromium are confronted by the hazardous and environment-contaminating potential. In production and use, expensive effluent treatment methods are required in order to be able to comply with the required limiting values for chromate ions.
The mentioned disadvantages of passivating layers based on chromium led to intensive efforts to develop chromium-free corrosion inhibitors. For example, the DE 10 2006 039 633 A1 discloses a chromium-free corrosion inhibitor with a pH ranging from 1 to 3 which, aside from water and fluorocomplex ions of titanium and/or zirconium, contains also at least one corrosion inhibiting pigment and at least one organic polymer, which is water soluble or water dispersible in the pH range mentioned and which, as such, has a pH ranging from 1 to 3 at a concentration of 50% by weight in an aqueous solution. In addition, phosphate ions, manganese ions, magnesium ions, molybdate ions, tungstenate ions, zinc ions, cobalt ions, nickel ions, vanadium ions and iron Ions may be present. Said water dispersible organic polymer may contain carboxyl groups as functional groups.
The DE 10 2005 059 314 A1 discloses an acidic, chromium-free aqueous solution of a fluorocomplex of at least one element selected from the group comprising boron, silicon, titanium, zirconium and hafnium with a pH within the range of 2 to 5.5, one or more components selected from the group comprising tin ions and bismuth ions, buffer systems for a pH ranging from 2.5 to 5.5, aromatic carboxylic acids with at least two groups, which contain donor atoms, or derivatives of such carboxylic acids being present in addition. As carboxylic acids, phthalic acid, salicylic acid, o-aminobenzoic acid and o-nitrobenzoic acid are mentioned.
The DE 30 31 270 A1 discloses an aqueous, acidic bath for passivating a metal substrate with an organic activation agent, wherein the organic activation agent is selected from the group of carboxylic acids, bath-soluble derivatives thereof and combinations thereof, in combination with one or more bath-soluble, film-forming materials, of which at least one chromium-free film-forming material is from the group of fluoride salts, oxalate salts, malonate salts, succinate salts and combinations thereof, wherein the chromium-free, film-forming material includes a film-forming element, which is not chromium and which forms a thin, adherent, coherent, hydrophobic, passivating coating on the metal substrate, wherein the film-forming element may be aluminum, silicon, titanium, vanadium, iron, cobalt, molybdenum, cerium and combinations thereof. As carboxylic acid, polyhydroxy carboxylic acids and polycarboxylic acids are named.
The search for a chromium-free corrosion inhibitor is made difficult by the fact that the passivating layer must have a coloration. It is necessary to produce a colored layer in order to be able to check the production in industrial use quickly, simply and visually. Only in this way is it possible to evaluate the quality of the coating without an expensive testing method and to ensure a frictionless, trouble-free course of the production.
Therefore, there continues to be a need for chromium-free compositions for coating metal substrates, which bring about sufficient protection against corrosion and, in addition, produce coloration on the metal surface, which can easily be verified visually.